(1) Field of the Invention
The present invention relates to the manufacture of strands formed of cut thermoplastic filaments, particularly glass filaments and more particularly still to a cutting method and assembly intended for such manufacture.
(2) Description of Related Art
As is known, the cutting of strands (sometimes known as threads) is obtained by passing the filamentary material between a wheel known as an anvil and a wheel known as a cutting wheel which is equipped with blades protruding uniformly at its periphery, the two wheels rotating in opposite directions to each other. The anvil wheel, generally made of elastomer of the polyurethane type, is arranged so that it comes into contact under pressure with the edge of the blades protruding from the cutting wheel, thus defining a cutting region.
To keep such installations at an optimum level of profitability, it is, of course, necessary for cutting to be performed continuously, with the fewest possible stoppages. Stoppages are essentially brought about by the breakage of the supply thread. After each breakage, the cutting device needs to be “restarted”, that is to say that the thread has to be placed back in the cutting region, with greater or lesser success.
There are several known solutions: Document FR 2 162 068 describes a cutting machine the press cylinder of which is extended by a frustoconical part. The machine is enclosed in a protective casing provided with an opening through which the frustoconical part of the press cylinder protrudes, the front face of which is itself hidden by a protective plate fixed to said casing. Between the plate and the casing there is thus a lateral opening through which the strand made up of glass filaments is brought by hand into contact with the frustoconical part of the press cylinder. The strand becomes wound onto and rises up along the conical surface before being taken between the cutter and the anvil.
The operation of such a device entails, on each restart, bringing the rate of travel of the strand back up to a value of about 1 to 2 m/sec: this is acceptable when the device is supplied with a strand from a single die, but is not so in the case of the simultaneous cutting of several strands from several dies. What happens is that any accidental breakage of one of the strands leads to stoppage of the forming operation on all the dies, and this disrupts their operation, accelerates wear of the hardware and appreciably reduces the productivity of the installation.
An alternative form of the previous device is described in French publication FR 2 204 715. According to the latter, the cutting machine comprises a press cylinder extended by a cone frustum ending in a small-diameter vertex. A start-up roll, consisting of a series of disks, presses on a generatrix of the cone frustum. The cutter/anvil assembly rotates constantly at its normal speed.
During a restart operation, the strand, passing first of all through a guide wheel aligned with the cutting region, is brought up close to the vertex of the cone frustum onto which it winds therefore at a low speed. Because the strand tends to follow a straight path, it soon becomes jammed between the start-up roll and the surface of the cone frustum, rises gradually up along the latter, experiencing an increasing drawing speed before finally being taken between the anvil and the cutter.
This arrangement therefore allows a strand to be introduced without altering the rotational speed of the cutting machine. However, this device is not without drawbacks.
Indeed, to avoid premature breaking of the strand, it is necessary for its drawing speed to increase gradually and this entails fitting to the anvil a cone frustum which is very long and therefore naturally mounted with overhang. This situation is exacerbated by the pressure exerted on the cone frustum by the series of disks that form the start-up roll. A device such as this wears very quickly particularly when drawing rates of between 30 and 50 m/sec are to be maintained.
Another type of device, described in document FR 2 397 370 also anticipates introducing the strand laterally into the cutting apparatus.
This device comprises an auxiliary press cylinder arranged next to one of the ends of the main press cylinder and aligned axially with the latter, and an auxiliary blade-holder cylinder arranged to collaborate with the auxiliary press cylinder.
Upon accidental breakage of a strand, the latter may be reintroduced into the cutting machine, without stopping the latter, as follows:
After manual drawing, the strand is introduced between the auxiliary cylinders at low speed, and held in this position by a grooved pulley.
The speed of the two auxiliary cylinders, which act like a cutting machine, is gradually increased until the strand reaches its normal drawing speed. By moving the grooved pulley, the strand is then brought into the main cutting region.
This device constitutes an interesting solution to the difficulties associated with introducing a thread-like strand into a cutting machine rotating at high speed. However, each time it is necessary to change the main cutting machine, the auxiliary cylinders have to be removed and refitted, and this is not without its disadvantages in continuous manufacture which demands a minimum of down time.
One solution to this problem is described in document FR 2 490 251 which essentially consists in drawing the strand in a region lying between the planes passing between the flanks of the blade-holder cylinder collaborating with an anvil cylinder, the axes of rotation of said cylinders passing through a roughly horizontal plane, then in deflecting the strand thus drawn toward the cutting region defined by the region of contact between the anvil cylinder and the blade-holder cylinder, to press it onto at least a portion of the surface of the cylinder arranged upstream, said surface portion being more or less adjacent to the cutting region.
However, this solution entails at least one human intervention for performing the initial drawing of the strand in a horizontal plane as far as the start-up region. It is at this stage that there are a great many breakages and therefore a great amount of time lost. This is especially true since several strands from several dies are generally brought by the operator to the starter together.
Thus, it has been possible to measure a breakage rate of the order of 20 breakages per die per day.
Another solution consists in conceiving of a cutting system such as described in U.S. Pat. No. 5,935,289 in which the strand is gripped under each die and brought automatically toward a restart system.
However, this system is sophisticated. In particular, it comprises a fairly complex restart assembly, guidance, protection and controls for the gripper which are all just as complex.
The present invention proposes an assembly which comprises neither a restart wheel nor a small guide roller and which simply by moving the strand in a straight line allows restarting to be performed automatically and which therefore proves far simpler to operate than the previous systems, making it possible to obtain a restart success rate of close to 100%, and in any event higher than 98%.
Furthermore, by comparison with a cutting assembly as described for example in Patent FR 2 490 251, the present invention is completely automated because it involves no human intervention on the strand.
Furthermore, the present invention has neither an arm for engaging the strand in the cutting region nor a restart wheel. These elements are essential to the operation of the prior art, but give rise to a not insignificant restart failure rate because of the forces applied to the strand as they are set in operation.
This is a major disadvantage of the prior art that the present invention is able to overcome.